The invention relates to a process for catalytic cracking more specifically making it possible to treat heavy hydrocarbon oils, as well as to the apparatus permitting the performance of this process.
The evolution of the nature of crude oil supplies, as well as the reduction in demand for products refined into fuel oils compared with lighter products of the petroleum type have led the oil refining industry to develop various processes making it possible to valorize natural heavy oils, as well as residual oils such as atmospheric distillation residues or vacuum distillation residues.
With respect to the catalytic cracking processes using such charges, it has been found that the main difficulties are due to the fact that these charges contain organic materials such as asphaltenes, as well as condensed polycyclic aromatic hydrocarbons, which are difficult to crack into lighter molecules able to provide high yields of fractions boiling at lower boiling points, but which tend to associate in order to give rise to large coke formations, which are deposited on the catalysts used and reduce their activity. Moreover, the presence in these heavy oils of large quantities of heavy metals, such as e.g. nickel, vanadium, iron, etc., is considered to be prejudicial, because these metals tend to poison or at least deactivate the zeolite-type catalysts generally used in existing fluid catalytic cracking processes, referred to as F.C.C.
A certain number of essential factors can still make it possible to convert heavy products into lighter fractions with a good selectivity, while keeping coke formation to a minimum.
Among these factors, one of the most important consists of ensuring, with respect to the contacting of the hydrocarbon charge (which is generally preheated and to which is optionally added steam) and the regenerated hot catalyst used in the catalytic cracking unit, a mixture such that the heat transfer between the charge and the catalyst is carried out as rapidly as possible and as regularly as possible. It is also vital for the renewal of the regenerated catalyst with respect to the introduction of the charge into the reaction zone is carried out permanently and efficiently, while in particular avoiding retro-mixing phenomena, which increase the contact times and lead to a significant reduction in the formation of light fractions by increasing the percentage by weight of coke formed on the catalyst grains.
Another important phenomenon, which is also prejudicial to the satisfactory operation of a F.C.C. unit, more particularly intended for the treatment of heavy charges, involves of the difficulty of obtaining a good radial homogeneity of the catalyst, both at the start of the reaction zone and all along said zone.
With regards to the selectivity of the cracking reactions in general terms, it is known that it improves as the flow of fluids approaches a piston-type flow, which is particularly difficult to obtain in conventional F.C.C. units.